The present invention relates to a method for producing asymmetric polymers. More particularly, this invention relates to a method for producing asymmetric polymers having vinyl aromatic hydrocarbon-conjugated diene copolymer arms wherein the vinyl aromatic hydrocarbon blocks of a controlled portion of the arms differ in molecular weight.
Heretofore, several methods have been proposed for preparing asymmetric radial polymers. As is well known in the prior art, linear coupled polymers are comprised of two arms coupled in the center by a coupling agent and radial polymers comprise three or more arms extending outwardly from a coupling agent nucleus. The known asymmetric polymers contain arms of at least two different polymers, which polymers may vary as to chemical composition, structure and/or molecular weight. A principle difference in the methods frequently used to prepare asymmetric polymers resides in the selection of a coupling agent which forms the center or nucleus of the polymer. Multifunctional coupling agents such as silicon tetrachloride have been used to form asymmetric radial polymers having three or four arms. Star-shaped radial polymers having many more arms have been formed using a poly alkenyl aromatic compound, such as divinyl benzene, as the coupling agent as described in Canadian Patent 716,645 and U.S. Pat. No. 3,985,830. Linear coupled polymers are formed using difunctional coupling agents such as dibromoethane.
Heretofore, linear coupled polymers were made by coupling two identical block copolymer arms, A-B (where A is a vinyl aromatic hydrocarbon and B is a conjugated diene) to form a symmetrical triblock polymer, A-B-A. Generally, asymmetric radial polymers referred to in the prior art are asymmetric in the sense that there are at least two different types of arms, one of which is an A-B arm and the other is either a B or an A arm, i.e., homopolymer polydiene or homopolymer poly vinyl aromatic hydrocarbon. The present invention provides a method for making asymmetric polymers which have at least two sets of A-B block copolymer arms wherein the A blocks of the block copolymer arms have differing molecular weights. Such polymers have the advantage of having a relatively high styrene content while at the same time maintaining a soft, tacky character. Further, the A blocks of differing molecular weight are made in a single reactor. Therefore, the process of the present invention also has the advantage of requiring a reduced amount of process equipment.